The present invention involves the post-manufacture treatment of fluffy carbon black to densify and pelletize it. More specifically, it involves a method and apparatus for increasing bulk or pour density of carbon black.
In several prior art methods of densifying carbon black the physico-chemical properties are changed significantly so that rubber manufacturer's specifications can not be met and the densification is unsuccessful economically. These methods involve varying parameters within the furnace itself or else involve changing from one type pin mixer to another in the pelletizing system. Also, various binders have been used in the pelletizer and somewhat cumbersome and inefficient control is obtained by changing pin mixers or binders.
The use of a micropulverizer on dry carbon black is old in the art as revealed by U.S. Pat. Nos. 2,843,874 and 2,917,374 which disclose a micropulverizer attached to a surge tank to apply work to dry fluffy carbon black which is received from a collection system. No method of control is shown for the micropulverizer.
Wet milling of carbon black to reduce oil absorption is disclosed in Japan Pat. No. 36,9170. The entire moisture content of the black is added prior to the wet milling. No system of control is given for maintaining constant levels of moisture.
The present invention differs from the above processes by simultaneously wetting and micropulverizing fluffy carbon black using a portion of the wetting material normally added in pelletization. The present invention also adds the wetting fluid directly to the carbon black in the micropulverizer rather than prewetting and includes controls for maintaining a constant ratio of carbon black to wetting fluid. The present invention may be used in conjunction with carbon black pelletization.
Carbon black particles are believed to be constructed of several carbon nuclei agglomerated together into a chain-like particle or structure. Some of the nuclei are apparently fused strongly together and some appear to be loosely joined by weak forces such as Van der Waal's forces. The structure arising from the strongly fused nuclei is very resistant to breakage and can be destroyed only by severe physical working. This type structure is referred to as persistent structure. The numerous carbon nuclei and persistent-structured agglomerates which are held to each other by the weaker forces of attraction constitute what is termed transient structure. Transient structure can be broken down into carbon nuclei and individual persistent structures by physical working which is much less severe than that required to break down persistent structure.
It has been found that the density of a macroscopic quantity of carbon black is dependent in part upon the degree of structure of the black particles. High-structured black tends to be less dense than does low-structured black as a result of the high incidence of voids in high-structured black caused by the long and irregularly shaped high-structure particle.
It is well known to use a physical milling process to increase the density of carbon black prior to pelletization although dry milling achieves very little reduction in structure.
A reduction of the transient structure has little effect on the physico-chemical properties of the black other than pour density, DBP and specific volume, but a reduction of the persistent structure results in a drastic change in many of these properties.
While carbon blacks can be used in most products without undergoing any reduction in transient structure, it is uneconomical and impractical to convey, store, and use the black in this highly fluffy and deflocculated state. It is difficult to package and results in contamination of the manufacturing facilities.
Thus, in retrospect, it is believed that an almost total reduction in the transient structure of carbon black without affecting persistent structure results in obtaining maximum pour density for the black without changing any of its other significant physico-chemical properties. Many of the known methods of physical working of carbon black result in not only breaking the transient structure, but also, because of the severe physical working involved, destroy the persistent structure and adversely degrade the desirable properties of the black. One such property which is degraded is the reinforcing ability of the black in rubber which appears to be highly dependent upon the degree of persistent structure present in the black.
In the prior art, the common method of densifying is by wet pelleting in a pin mixer type pelletizer. This changes the fluffy black into a pill form but significant reduction in transient structure in the black can only be accomplished by operating the pin mixer at extremely high rotational speeds in order to apply the needed amount of work to the black or using more mixers or pins. This results in damage to the machinery and unsatisfactory pellets. It also requires a great expenditure of energy to rotate the pin mixer shafts at such a high speed.
Another deficiency of this type of process is the method of control of carbon black wetness. When using the prior art methods, the operator of the pin mixer takes samples of the finished pellets as they are emitted from the end of the pin mixer and tests them for moisture content. The time elapsing between the instant that particular black was wetted and added to the pelletizer and the time the operator can purge the pin mixer and obtain a test result for final pellet moisture content may be as long as 11/2 or 2 hours which means that if the moisture content is wrong, production of unacceptable carbon black pellets has proceeded for possibly 2 hours, resulting in much wasted time and materials. In other words, the time lag in this feedback loop is 11/2 to 2 hours, as compared to a few seconds for that of the present invention where errors in the moisture level of the black are detected and corrected almost instantaneously.
Also, in the present invention, due to the fact that by the time the black reaches the pelletizer it has already been wetted with a portion of the fluid normally added in pelletizing, the remaining liquid is accepted more readily and more even wetting of the black is the result. Just as a damp sponge absorbs water more easily than does a dry one, so does damp carbon black absorb a fluid more easily than dry carbon black. This means a better pellet is formed and pellet size distribution is in a very desirable, narrow range.